HVDC (high-voltage direct current) electrical power transmission uses direct current for the transmission of electrical power. This is an alternative to alternating current electrical power transmission which is more common. There are a number of benefits to using HVDC electrical power transmission. HVDC is particularly useful for power transmission over long distances and/or interconnecting alternating current (AC) networks that operate at different frequencies. A first station may therefore transmit electrical energy to a second station over a DC transmission line, e.g. an overhead line or subsea or buried cable. The first station may generate the DC supply by conversion from a received AC input supply. The second station then typically provides conversion back from DC to AC. Each of the first and second stations may therefore typically comprise a converter for converting from AC to DC or vice versa.
Initially HVDC power transmission systems tended to be implemented for point-to-point transmission, i.e. just from the first station to the second station. Increasingly however it is being proposed to implement HVDC power transmission on a mesh-network or DC grid comprising a plurality of DC transmission paths connecting more than two voltage converters. Such DC networks are useful, for example, in applications such as electrical power generation from renewable sources such as wind farms where there may be a plurality of sources that may be geographically remote.
To date most HVDC transmission systems have been based on line commutated converters (LCCs), for example such as a six-pulse bridge converter using thyristor valves. LCCs use elements such as thyristors that can be turned on by appropriate trigger signals and remain conducting as long as they are forward biased. In LCCs the converter relies on the connected AC voltage to provide commutation from one valve to another.
Increasingly however voltage source converters (VSCs) are being proposed for use in HVDC transmission. HVDCs use switching elements such as insulated-gate bipolar transistors (IGBTs) that can be controllably turned on and turned off independently of any connected AC system. VSCs are thus sometime referred to as self-commutating converters.
Various designs are VSC are known. In one form of known VSC, often referred to as a six pulse bridge, each valve connecting an AC terminal to a DC terminal comprises a set of series connected switching elements, typically IGBTs, each IGBT connected with an antiparallel diode. The IGBTs of the valve are switched together to connect or disconnect the relevant AC and DC terminals, with the valves of a given phase limb being switched in antiphase. By using a pulse width modulated (PWM) type switching scheme for each arm, conversion between AC and DC voltage can be achieved.
In another known type of VSC, referred to a modular multilevel converter (MMC), each valve comprises a series of cells connected in series, each cell comprising an energy storage element, such as a capacitor, and a switch arrangement that can be controlled so as to either connect the energy storage element in series between the terminals of the cell or bypass the energy storage element. The cells are often referred to as sub-modules with a plurality of cells forming a valve module. The sub-modules of a valve are controlled to connect or bypass their respective energy storage element at different times so as to vary over the time the voltage difference across the valve. By using a relatively large number of sub-modules and timing the switching appropriately the valve can synthesize a stepped waveform that approximates to a sine wave and which contain low level of harmonic distortion.
In normal use the VSCs of the HVDC stations are typically controlled with reference to the AC waveform of the relevant connected AC network to achieve a desired power flow. Thus, the AC waveform is used as an external reference for controlling switching of the VSC.
At times, there may be a need to start-up at least part of such a power transmission/distribution network, for instance on first initialization of a new AC or DC network or following a power outage or blackout in an existing AC network. In such case, there may be no existing AC waveform for the VSC control to use as an external reference and the normal control of the VSC may not function correctly until a sufficiently stable AC voltage has been provided.
Usually starting or restarting an AC network, which is sometimes referred to as a “black start”, requires starting the voltage generators connected to the AC network in sequence with one generator supplying power for the next generator. For example, a diesel generator may be used to supply local power to a power generation station. This local power generation may energize at least part of the AC network to a sufficient extent to enable another power generator to be enabled and so on until the network has reached a stable level, where normal control may be enabled. Any connected HVDC stations are conventionally started and energized from the AC system.
It has been proposed however that a VSC connected to an AC network can be used to start-up an AC network. Thus, if a first VSC is connected to a first AC network which is dead and the first VSC can receive power via a DC link from a second VSC, which may be connected to a second, functioning AC network, the first VSC may be operated to start-up the first AC network. The first VSC can be used as a voltage generator to generate an AC voltage within the first AC network.
A first VSC connected to a dead AC network may be used to generate an AC waveform for re-starting the dead AC network, the first VSC receiving power via a DC link from a second VSC connected to a functioning AC network. The second VSC maintains the voltage of the DC link at the nominal voltage of the DC link.